With me (and that shirt in particular) it has always been the men pointing out they have been staring at my chest, but you know, embarrassedly saying "It's the image on your shirt." This has happened with five separate men, one being a total stranger, one having apologized for it more than once. To the stranger I was like "sure dude, it's the T-shirt." To all the other men, they've all been friends and I 've never had any inkling of piggishness from them. For all but one of them, it's probably the only time I've even felt a gender divide of any kind. It's an easy-to-stare-at image, and then I think the men who do (always in a work environment, in an off-focused, far away state that they snap out of) have a split second where they realize there are tits behind that t-shirt and they they've crossed their own arbitrary line of how much time one should stare at a woman's chestular area, and get all embarrassed and have to explain. I've actually had the same issues with a Gandhi t-shirt I have, which leads to the weirdest men needing to let me know they really like Gandhi. "No, cool shirt! I really like Gandhi."
I am almost totally oblivious to tit-staring, and it's only when dudes I know get all embarrassed that I realize it may have been happening.